The present invention relates to shielding for electrically conductive cables, and more particularly, to a device and method for resisting the transfer of electromagnetic interference (EMI) to and from an electrically conductive core within a cable covered with a mesh of aluminum wire that is protected from corrosion.
Electrically conductive cables that carry signals among electrical components may be subjected to unwanted EMI from various external sources and may spray unwanted EMI to other nearby components. EMI may introduce unwanted, spurious signals into the cables or into the other components and includes interference from across the electromagnetic spectrum, including that found at radio frequencies. It is known to shield such cables so that spurious signals caused by the EMI are eliminated or reduced to acceptable levels. EMI shields operate by converting received electromagnetic energy into a current that is carried to a ground by the shield. To this end, the shield desirably has low electrical resistance and makes a secure contact with a connector to ground.
A cable 10 typical of the prior art is illustrated in FIGS. 1 through 4. As seen in FIGS. 1 and 2, a shield 12 may form a generally tubular shape surrounding an electrically conductive core 14. The core 14 may consist of one or more longitudinally extended wires 16 that may be loose singles, twisted pairs, shielded twisted pairs or coaxial cables as is known in the art. The core 14 may be covered with an insulative layer 18 or the wires in the core 14 may be individually insulated 20. The cable 10 may have connectors 22 for connecting the core 14 to electrical components (not shown) and for connecting the shield 12 to a ground. The connectors 22 have a backshell 24 that underlies an end portion 26 of the shield 12, and a strap 28 that overlies the end portion 26 and the backshell 24 to compressibly hold the shield 12 in place and electrically connect the shield 12 to the connector 22 (and thereby to a ground) as is known in the art. Other connecting means may be used as appropriate for the particular cable application.
With reference now to FIGS. 3 and 4, the shield 12 may be woven in a variety of patterns, with a braid being preferred (by way of example, a herring bone pattern is shown). The pattern may be created by weaving individual wire strands 30 onto the insulated core 14 in multi-stranded ribbons 32. Various techniques for weaving the wires are known and may include weaving machines having numerous (e.g., twenty four) spools of wire 30 that are woven onto the core. Typically, each of the spools has on it the number of strands being used to form a ribbon 32 so that each ribbon 32 is woven onto the core at one pass of the weaving machine. By way of example, a five-stranded ribbon 32 is illustrated in FIG. 4. Several layers of ribbons 32 may be woven onto the core.
The shielding effectiveness of the shield 12 is a measure (typically in db) of the change of EMI across the shield. As is known, shielding effectiveness is influenced by various factors, with the more significant being the number of layers of ribbons 32 in the shield 12, the braid angle (the angle A in FIG. 3) and the optical coverage (the portion of the circumference of the core 14 covered by the shield 12, the holes 34 not being covered in the illustrated examples). Shielding effectiveness improves with increased number of layers, smaller braid angle and increased optical coverage. Shielding effectiveness decreases as the frequency of the EMI increases (e.g., shield effectiveness is higher at 1 MHz than at 10 MHz.) Shielding effectiveness also increases as the electrical resistance associated with the shield 12 decreases. Such resistance is typically measured from the connector 22 at one end of the cable 10 to the connector 22 at the other end of the cable 10.
Various types of shields 12 are known and may be effective if one is willing to accept the weight added by the shield. For example, it is known that the wire 30 may be copper, or tin plated copper. However, in many applications, such as in moving vehicles (e.g., automobiles, aircraft, ships), the weight of the shield must be considered. Copper wire is relatively heavy and may be replaced with lighter aluminum wire. Aluminum wire is almost as effective as copper wire in shields and provides the added benefits of increased flexibility and lower cost for the wire itself.
However, there are significant disadvantages to using aluminum wire in cable shields in certain environments that heretofore have not been successfully overcome. The primary obstacle to the use of aluminum wire in cable shields is that aluminum corrodes easily. Bare aluminum oxidizes upon exposure to the air causing increased resistance, thereby reducing the effectiveness of a shield made of aluminum wire. Further, when aluminum contacts a dissimilar metal (e.g., copper, nickel, brass, stainless steel, silver) a galvanic reaction occurs that causes the aluminum to corrode more rapidly. Prolonged exposure to adverse environments, such as maritime environments, may reduce shielding effectiveness and make aluminum an unacceptable shielding material. In addition, the product of the corrosion, aluminum oxide, is powdery and highly abrasive, thereby hastening disintegration of the wire due to fretting (rubbing one wire against another).
A further problem with aluminum wire is that it is compressible and, when compressed, tends to flow away from the compressed area (a phenomena known as cold flow). In shielded cables such as illustrated in FIG. 1, the backshell 24 and the strap 28 compress the end portion 26 of the shield 12 causing cold flow. When the shield cold flows, the connection of the shield to the connector is degraded and the resistance increases between the shield 12 and the connector 22. As discussed above, increased resistance reduces shielding effectiveness.
Accordingly, it is an object of the present invention to provide a novel device and method for shielding an electrically conductive cable that obviates the problems of the prior art.
It is a further object of the present invention to provide a novel device and method for shielding an electrically conductive core with a mesh of aluminum wire that has been treated with a chromate conversion coating to resist corrosion.
It is yet a further object of the present invention to provide a novel device and method for reducing EMI transferred to and from an electrically conductive core in a cable shielded with an aluminum wire mesh in which the cold flow problem has been alleviated.
It is another object of the present invention to provide a novel device and method for reducing EMI through the use of a mix of tin plated copper wire and aluminum wire treated with a chromate conversion coating.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of preferred embodiments.